(1) Field of the Invention
The present invention relates to high-strength, high-modulus, pitch-based carbon or graphite fibers having a novel peculiar internal structure and a process for the preparation thereof.
(2) Description of the Related Art
The present invention relates to a process for the preparation of high-strength high-modulus pitch type carbon fibers having a novel peculiar internal structure.
At the beginning, carbon fibers were prepared from rayon, but in view of the characteristics and from the economical viewpoint, at present, the carbon fibers used are mainly PAN type carbon fibers prepared from poly-acrylonitrile (PAN) fibers and pitch type carbon fibers prepared from coal or petroleum pitches. In particular, the technique of forming high-performance carbon fibers from a pitch has attracted attention in the art because this technique is advantageous from the economical viewpoint. For example, carbon fibers obtained by melt-spinning an optically anisotropic pitch and infusibilizing and carbonizing the obtained pitch fibers have higher strength and higher modulus than those of conventional pitch type carbon fibers.
It has been found that further improved physical properties can be manifested by controlling the internal sectional structure in pitch type carbon fibers (see Fuel, 1980, 60, 839 and Japanese Unexamined Patent Publication (Kokai) No. 59-53717 corresponding to U.K. Patent Application GB No. 2129825A).
As the sectional structure of pitch type carbon fibers, there can be mentioned a random structure, a radial structure, an onion structure, and a composite structure thereof. The radial structure is not preferred because cracks are readily formed and the physical properties are reduced by macro-defects. That is, the random structure in pitch type carbon fibers is practically a radial structure in which the lamella size is small, and this structure is preferred from the viewpoint of the strength, but since cracks are readily formed if drafting or quenching is insufficient at the step of preparing or spinning a pitch, the preparation conditions are considerably limited.
The onion structure is obtained by elevating the temperature of a pitch to a level higher than the temperature causing the change of the viscosity in the pitch and then spinning the pitch (see Japanese Unexamined Patent Publication (Kokai) No. 59-53717). However, in the case of an ordinary optically anisotropic pitch, the viscosity-changing temperature is higher than 350.degree. C., and therefore, the spinning stability is poor and the obtained fibers are likely to contain voids. Accordingly, it is difficult to obtain voidless fibers having an onion structure stably by melt spinning.